Instance helper to replace site title helper (#2038)

* Move site title helper to instance helper (name change only)

* Remove newline in <title> tag

* Add site_hostname helper method to wrap up local_domain value

* Use site_hostname helper in places that need local_domain value

app/views/about/more.html.haml

@@ -1,11 +1,11 @@
 - content_for :page_title do
-  #{Rails.configuration.x.local_domain}
+  = site_hostname
 
 .wrapper.thicc
   .sidebar-layout
     .main
       .panel
-        %h2= Rails.configuration.x.local_domain
+        %h2= site_hostname
 
         - unless @instance_presenter.site_description.blank?
           %p= @instance_presenter.site_description.html_safe